Curiosity on how Touch ID work

As we know, the Touch ID on iPhone 5s is incredible be used to unlock the phone. However, I have a question that how actually Touch ID actually work (as I do not have iPhone 5s right now).

When the phone sleep, can we just tab on Touch ID (without actually clicking on home button) and the phone will wake up and unlock by itself or we have to click on home button to wake the phone up first then tab our finger on Touch ID to unlock it.

It is kind of ridiculous question but for me, as conservative person (too much conservative sometimes), I want to keep the home button as long as possible.

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